Render image rendering incorrectly with a checkered pattern
What @Jachym Michal said is correct. Press ALT H in the viewport, your second object should appear, otherwise search in the outliner.  Btw. this irregular pattern of triangles and weird stripes is called z-fighting, where z is the distance to the camera, it means 2 faces occupy the same space and the camera has troubles deciding which one to show in each sample.